Owing to its excellent optical properties, mechanical properties and packaging adaptability, polypropylene film has been widely applied to the field of packaging material. In particular, biaxially oriented polypropylene film (BOPP) has been widely applied to package various products such as foods, medicines, commodities and cigarettes and also applied as a substrate material for high-strength composite film, due to its features of softness, good transparency, non-toxicity, good water repellent property and high mechanical strength.
Biaxially oriented polypropylene film (BOPP) is generally produced with polypropylene resin as raw material by the steps of: extruding the resin to obtain a film, and then subjecting the film to biaxial stretching, heat treatment and cooling treatment. To meet the requirement of high-speed film production in a large scale, the polypropylene resin raw material should have both good tensile strength and excellent high-temperature stretching property. As far as polypropylene is concerned, it is generally deemed that a lower isotacticity index and a lower melting temperature of the polymer are favorable to the processing and forming of the BOPP film, but the mechanical properties such as tensile strength and stiffness of the film as obtained will be reduced. Thus, a method of adding C5-C9 petroleum resin to isotactic polypropylene material was developed in order to increase modulus of the material, thereby improving the stiffness of the finally obtained polypropylene film. For instance, Chinese Patent CN1102419A disclosed a propylene polymer composition comprising C5 hydrocarbons free of polar group, the use of which can improve the steam barrier property, transparency and Young's modulus of the polypropylene oriented film obtained thereby. However, this method not only needs extra components thereby increasing the production cost, but also does not have a satisfactory result.
In the prior art, the methods for preparing BOPP films with propylene random copolymers, in particular, compositions comprising propylene random copolymers, as raw materials are also disclosed, wherein the random copolymers are generally copolymers of propylene and alpha-olefins such as ethylene. For instance, as disclosed in Chinese Patent CN1404500A, the amount of ethylene in propylene copolymer or in propylene polymer composition should be greater than 0.8 wt %, generally in the range of 0.8-1.5 wt %. Moreover, the MFR value of the first polypropylene (generally a homopolymer) is lower than that of the second propylene random copolymer, i.e., the molecular weight of the first polypropylene (generally a homopolymer) is higher than that of the second propylene random copolymer, and the difference in the MFR values being preferably greater than 5 g/10 min. Such control in ethylene content and melt flow rate in the propylene polymer composition might be applicable to the special polymerization reactor and special polymerization method, wherein two interconnected gaseous polymerization reaction zones are used, as adopted in the embodiment of that patent application. However, as to a common reactor or polymerization method, such control cannot balance the requirement of producing BOPP film and the achievement of polypropylene resin having satisfactory inherent properties, which mainly concern high soluble content of the polypropylene resin and reduced transparency and stiffness of the film.
In view of the situations in the prior art, it is desired to develop a propylene polymer composition for producing a biaxially oriented film, which can better balance the high-temperature stretching properties and physical properties of the film, thereby obtaining a BOPP film having excellent comprehensive properties.
The present inventors discovered by laborious experiments that by increasing the isotacticity of the polymer having a relatively higher MFR value (i.e., a relatively lower molecular weight) in the propylene polymer composition to keep the overall isotacticity of the propylene polymer composition at a relatively higher level, a polypropylene oriented film having high modulus, with its other properties such as transparency and mechanical properties being somewhat improved, can be obtained, without the addition of any stiffening agent (such as C5-C9 petroleum resin)